The resolution of Analog To Digital Converters, or ADCs, as measured by their effective number of bits (ENOB) is limited by their “aperture-jitter” at high sampling rates. Photonic time-stretching allows effective compression of the analog-input bandwidth, so that quantizers that sample at much lower rates fi can be used to accomplish A/D conversion with high resolution.
As documented in W. Ng, T. Rockwood, G. Sefler, G. Valley: “Demonstration of a Large Stretch-Ratio (M=41) Photonic Analog-to-Digital Converter with 8 ENOB for an Input Signal Bandwidth of 10 GHz”, IEEE Photonic Technology Letters, Vol. 24 (14), 1185-1187 (2012), an ENOB >8 could be attained for fsig=10 GHz with a Photonic Time Stretch ADC.
As illustrated in FIG. 1, a photonic TS ADC 10 comprises a source of light 12, such as a mode-locked laser, for generating a series of pulses having each a broad (˜30-40 nm) supercontinuum (SC) spectrum.
FIG. 2 is a time-domain illustration of a series of pulses 13 comprising each a plurality of wavelengths. In FIG. 2, each exemplary pulse of light comprises four wavelengths of light represented on the figure by the symbols “Δ”, “□”, “◯”, and “*”.
Returning to FIG. 1, ADC 10 comprises a first dispersion element 14 that chirps each pulse 13 by spreading in time the wavelengths comprised in the pulse. Dispersion element 14 can be made of a material the refractive index of which varies with wavelength, such that the wavelengths of the pulse traverse the material at different speeds and exit the material at different times. Dispersion element 14 is arranged so that the time-stretched optical pulses overlap with each other and dispersion element 14 outputs a continuous optical signal 16, comprised of wavelengths that periodically vary with time.
FIG. 3 is a time-domain illustration of the continuous optical signal 16 comprised of the overlapping juxtaposition of the time-spread optical pulses 13. Continuous optical signal 16 is comprised of various wavelengths, represented on the figure by the symbols “Δ”, “□, “◯”, and “*”, that periodically vary with time. The wavelength of a pulse 13 that gets out of dispersion element 14 the faster (represented by symbol “Δ” in FIG. 3) overlaps with the wavelength of the next pulse 13 that gets out of dispersion element 14 the slower (represented by symbol “*” in FIG. 3).
Returning to FIG. 1, ADC 10 comprises an electro-optic modulator 18 arranged for modulating continuous optical signal 16 with an input analog signal 20 into a modulated optical signal 22.
FIGS. 4 and 5 are time-domain illustrations of input analog signal 20 and modulated optical signal 22.
Returning to FIG. 1, ADC 10 comprises a time-controlled demultiplexer 24 arranged for separating the modulated optical signal 22 into a plurality of modulated optical signal segments 26. ADC 10 further comprises a plurality of second dispersion elements 28, coupled each to an output of demultiplexer 24, for spreading in time the wavelengths comprised in each modulated optical signal segment 26.
FIG. 6 is a time-domain illustration of a time-spread modulated optical signal segment 30 as output by a second dispersion element 28.
Returning to FIG. 1, ADC 10 comprises, coupled to the output of each second dispersion element 28, a sampler 32 arranged for sampling the time-spread modulated optical signal segments 30.
FIG. 7 is a time-domain illustration of a series of samples 34 obtained by sampling the time-spread modulated optical signal segment 30 as output by a second dispersion element 28.
Returning to FIG. 1, ADC 10 comprises a calculator 36 arranged for receiving the samples 34 output by each sampler 32, and for constructing a digitized image of the input analog signal 20 based on the samples 34.
If D1 is the dispersion coefficient (given in psec/nm) of first dispersion element 14 and D2 the dispersion coefficient of each dispersion element 28, the stretch ratio M of ADC 10 is given by: M=1+D2/D1. However, the number of channels in output of demultiplexer 24, which is needed to de-serialize an input-signal of continuous time-duration (CT) in ADC 10, is directly related to M. It follows that a stretch-factor M of 20 typically requires 20 or more parallel channels to de-serialize the signal. This, in turn, increases the size, weight and power consumption (SWAP) of the ADC.
There exists a need for a high resolution ADC having reduced size, weight and power consumption with respect to ADC 10.